COLUMBIA, S.C. - Republican Jon Huntsman's endorsement of Mitt Romney further cements Romney as the favorite in Saturday's pivotal South Carolina primary, adding another name to the list of party figures calling him best able to beat Democratic President Bararck Obama.

However, it leaves Romney's more conservative rivals still fighting with one another to emerge as his strongest challenger. And it prompted at least one backer of Texas Gov. Rick Perry to urge him to quit the race.

Keeping race civil

Besides offering Romney his support, Huntsman also appealed for civility in a race that has become defined by attack advertisements paid for by anonymous groups.

"At its core, the Republican Party is a party of ideas, but the current toxic form of our political discourse does not help our cause," he said.

Huntsman's endorsement offers little in the way of a campaign organization or bankable support to Romney.

But it does add to the sense of inevitability the former Massachusetts governor has worked to portray in a state that has voted for the eventual GOP since 1980 and where he is campaigning with backing from the state's new Gov. Nikki Haley and Arizona Sen. John McCain, winner of the 2008 primary here.

Romney stands a good chance of winning the votes Huntsman would have received. Both tout business backgrounds and are social moderates in a state where social conservatives are an influential bloc. Polls show Romney was most often the second choice of Huntsman backers.

"Certainly, it will help Gov. Romney here, it's just not clear how much," said former state Attorney General Henry McMaster. An early Huntsman supporter, McMaster had not committed to another candidate after Huntsman's announcement.

Unanswered question

But Huntsman's leaves unanswered the overriding question of the Republican campaign: whether voters looking for a candidate more conservative than Romney will unite behind Rick Santorum, Newt Gingrich or Perry and send a strong challenger to compete against Romney in the Florida primary or beyond.

Santorum, a former Pennsylvania senator, dismissed the endorsement during a campaign stop at a Columbia restaurant. "Moderates are backing moderates," he said. "No surprise there."

Gingrich, the former House speaker locked in a bitter battle with Romney, said at an appearance in Myrtle Beach: "Why would you want to nominate the guy who lost to the guy who lost to Obama?"